CCRs of the above kind are known from e.g. US 2010/283400. A CCR's goal is to regulate a current output level around a working point which can e.g. be set by an operator or an airport control tower. The current output level should be as constant as possible near to a pre-set working point in order to ensure that the airfield lights emit at constant luminous intensity. Since it is desirable to use different intensities depending on the circumstances (e.g. day or night, good or low visibility), the working point can change and the CCR must be able to adapt the current output level accordingly.
Airfield lights are typically arranged in long series connections. The length and number of lights in such series and hence the power to be delivered to it can differ between airports, between the type of lights and their use. CCR manufacturers therefore need to provide a wide range of CCRs, each corresponding to a particular input line voltage and frequency, output current and equipment power. This leads to a large number of different CCR product items, which increases stocks, manufacturing resources and cost.
CCR reliability is another aspect, since it is an important safety issue. It is known to have complete backup CCR units installed at airports, which requires a huge additional space and leads to a significant cost increase.
An attempt to tackle the reliability issue is described in US 2010/026207 by providing a twin voltage conversion unit connected in cascade to a common transformer operating at grid frequency (50/60 Hz). The idea here is to add a redundant (non-operating) voltage conversion unit, which is easily replaceable. In the event of a failure, the operating and non-operating voltage conversion units can be switched automatically, such that the broken part can be replaced later during a maintenance service.
The same document describes that it is not excluded to use the two voltage conversion units contemporarily for the operation of the CCR, but fails to teach how the co-operation is to be carried out. Additionally, the document describes a common control unit for controlling operation of the voltage conversion units and for activating switches configured to activate and deactivate one or the other voltage conversion unit.
The system of US 2010/026207 is more reliable, but at an increased expense per CCR unit, since each CCR unit will now have an additional voltage conversion unit identical to the one already present.
Hu Wei et al., in “Input-series output-parallel AC/AC converter”, 2010 5th IEEE Conference on industrial electronics and applications, 1 Jun. 2010, pages 1018-1022 describes a two-level AC/AC converter with a high frequency link, which can be used in applications with high input voltage and relatively low output voltage. Such a circuit is not applicable to airfield lighting installations due to its limited power. Furthermore, a disadvantage is that it is not reliable, since operation of both levels is required for obtaining a desired output.
EP 1063758 describes a power supply device comprising a plurality of resonance type switching converters arranged in parallel. Such devices allow for supplying a large power to a load. However, the described devices feature a DC output voltage, which is not workable for an airfield lighting installation, since airfield lighting installations must always be provided with AC current for corrosion protection of the cables and connectors, and in order to be able to use transformers to connect the lights to the supply line for lightning protection, independently of the type of lights (incandescent or LEDs).
US 2002/0074862 describes a parallel power source system comprising a plurality of power supply units, so designed that even when one power supply unit fails, the other power supply unit can ensure power supply. The described system, however, generates DC outputs from AC input supply voltages and cannot therefore be used for supplying airfield lighting installations. Each power supply unit comprises a high frequency transformer supplied with a high frequency pulse current to its primary coil.